This invention relates to a cathode assembly and more particularly, to a cathode assembly for an electric arc spray apparatus which is designed to provide more secure cathode retention, improved heat transfer management and less cathode erosion, thereby increasing the life of the cathode relative to prior cathode designs.
Electric arc spray apparatuses are used in various applications to apply protective coatings to surfaces such as metal, ceramic and composite surfaces. One type of electric arc spray apparatus includes a cathode and a constricting nozzle which is disposed around the cathode and which emits gas. The cathode is typically connected to a negative terminal of a power supply through a high frequency and/or high voltage generator, and is used to initiate an electrical arc between the cathode and the nozzle. A high velocity jet of gas is directed into a gap formed between the cathode and the nozzle. The gas is ionized and heated as it flows through the gap and is discharged through a small orifice in the nozzle. The orifice directs the heated plasma gas towards the tip of a feedstock metal wire. The plasma arc attaches to or xe2x80x9ctransfersxe2x80x9d to the metal wire, thereby melting the wire. The high velocity plasma jet disperses the molten metal into fine particles which form a spray stream which is directed upon a surface which is desired to be coated. Electric arc spray apparatuses are discussed and described for example in U.S. Pat. No. 5,808,270 of Marantz et al. which is assigned to the present assignee and which is fully and completely incorporated herein by reference.
The cathode used within electric arc spray apparatuses is typically made of tungsten or another durable thermionic emitting material. The cathode is fitted within a cathode holder which is made of another material such as copper or brass. Particularly, in prior apparatuses the cathode is typically retained within the holder by use of a swaging, pressing or brazing process. During operation, the cathode is heated to extremely high temperatures and the holder assists in dissipating heat from the cathode. After extended use, the cathode may crack due to the extreme temperatures and due to the limited heat dissipation provided by the cathode holder. Furthermore, the fitted cathode may also be displaced or expelled from the cathode holder due to repeated thermal cycling (i.e., repeated thermal expansion and retraction), thereby causing total failure of the plasma gun. These extreme conditions may also cause cathode erosion and other damage. As a result, the life of these prior cathodes is typically limited, and they must be replaced relatively frequently, thereby increasing production cost and decreasing efficiency.
There is therefore a need for a cathode assembly for a thermal spray gun which has improved heat transfer management, efficiency, and durability and which will increase the life of the cathode relative to prior designs.
A first non-limiting advantage of the invention is that it provides a cathode assembly for use with an electric arc spray apparatus which allows the cathode to have an increased diameter and a longer length for improved heat transfer characteristics, electrical contact and durability.
A second non-limiting advantage of the invention is that it utilizes a positive retention method and cathode design which results in better heat transfer management and less cathode erosion relative to prior cathode assemblies.
A third non-limiting advantage of the invention is that it includes a cathode retention device which prevents the cathode from being expelled from the cathode holder.
A fourth non-limiting advantage of the invention is that it allows the cathode to be relatively quickly and easily replaced.
According to a first aspect of the present invention, a cathode assembly is provided for use in an electric arc apparatus. The cathode assembly includes a cathode holder having a cathode retention cavity; a cathode having a first portion of a first diameter which is disposed within the cathode retention cavity, a second portion having a second diameter and which extends from the first portion, and a shoulder portion; and a retention member which is removably secured to the cathode holder and which includes a first aperture through which the second portion of the cathode extends and a first inner surface which engages the shoulder portion, effective to retain the cathode within the cavity.
According to a second aspect of the present invention, a method is provided for retaining a cathode within an electric arc spray apparatus. The method includes the steps of: providing a cathode holder having a cavity for holding the cathode; attaching the cathode holder to the electric arc spray apparatus; forming a shoulder portion on the cathode; fitting the cathode within the cavity such that a tip portion of the cathode extends from the cavity; providing a generally hollow retention member having a first aperture; and removably securing the retention member to the cathode holder, effective to cause the tip portion of the cathode to extend through the aperture and to cause a first surface of the retention member to engage the shoulder, thereby retaining the cathode within the cathode holder.
These and other features, aspects, and advantages of the invention will become apparent by reading the following specification and by reference to the following drawings.